arXiv subjects

Bharat Ratra

Publications and source records attributed to Bharat Ratra.

At least 73 records · Page 4Linked to original sources

arXiv2018

Planck 2015 constraints on the non-flat $Λ$CDM inflation model

We study Planck 2015 cosmic microwave background (CMB) anisotropy data using the energy density inhomogeneity power spectrum generated by quantum fluctuations during an early epoch of inflation in the non-flat $Λ$CDM model. Unlike earlier analyses of non-flat models, which assumed an inconsistent power-law power spectrum of energy density inhomogeneities, we find that the Planck 2015 data alone, and also in conjunction with baryon acoustic oscillation measurements, are reasonably well fit by a closed $Λ$CDM model in which spatial curvature contributes a few percent of the current cosmological energy density budget. In this model, the measured Hubble constant and non-relativistic matter density parameter are in good agreement with values determined using most other data. Depending on parameter values, the closed $Λ$CDM model has reduced power, relative to the tilted, spatially-flat $Λ$CDM case, and can partially alleviate the low multipole CMB temperature anisotropy deficit and can help partially reconcile the CMB anisotropy and weak lensing $σ_8$ constraints, at the expense of somewhat worsening the fit to higher multipole CMB temperature anisotropy data. Our results are interesting but tentative; a more thorough analysis is needed to properly gauge their significance.

astro-ph.CO↗

arXiv2018

First study of reionization in the Planck 2015 normalized closed $Λ$CDM inflation model

We study reionization in two non-flat $Λ$CDM inflation models that best fit the Planck 2015 cosmic microwave background anisotropy observations, ignoring or in conjunction with baryon acoustic oscillation distance measurements. We implement a principal component analysis (PCA) to estimate the uncertainties in the reionization history from a joint quasar-CMB dataset. A thorough Markov Chain Monte Carlo analysis is done over the parameter space of PCA modes for both non-flat $Λ$CDM inflation models as well as the original Planck 2016 tilted, spatially-flat $Λ$CDM inflation model. Although both flat and non-flat models can closely match the low-redshift ($z\lesssim6$) observations, we notice a possible tension between high-redshift ($z\sim8$) Lyman-$α$ emitter data and the non-flat models. This is solely due to the fact that the closed models have a relatively higher reionization optical depth compared to the flat one, which in turn demands more high-redshift ionizing sources and favors an extended reionization starting as early as $z\approx14$. We conclude that as opposed to flat-cosmology, for the non-flat cosmology models (i) the escape fraction needs steep redshift evolution and even unrealistically high values at some redshifts and (ii) most of the physical parameters require to have non-monotonic redshift evolution, especially apparent when Lyman-$α$ emitter data is included in the analysis.

astro-ph.CO↗

arXiv2018

Constraints on dark energy dynamics and spatial curvature from Hubble parameter and baryon acoustic oscillation data

We use all available baryon acoustic oscillation distance measurements and Hubble parameter data to constrain the cosmological constant $Λ$, dynamical dark energy, and spatial curvature in simple cosmological models. We find that the consensus spatially flat $Λ$CDM model provides a reasonable fit to the data, but depending on the Hubble constant prior and cosmological model, it can be a little more than 1$σ$ away from the best-fit model, which can favor mild dark energy dynamics or non-flat spatial hypersurfaces.

astro-ph.CO↗

arXiv2018

Median Statistics Estimate of the Galactic Rotational Velocity

We compile a complete collection of 19 recent (since 2000) measurements of $Θ_0$, the rotational velocity of the Milky Way at $R_0$ (the radial distance of the Sun from the Galactic center). These measurements use tracers that are believed to more accurately reflect the systematic rotation of the Milky Way. Unlike other recent compilations of $Θ_0$, our collection includes only independent measurements. We find that these 19 measurements are distributed in a mildly non-Gaussian fashion and a median statistics estimate indicates $Θ_0 = 220\ \pm 10$ km\,s$^{-1}$ (2$σ$ error) as the most reliable summary, at $R_0 = 8.0 \pm 0.3$ kpc (2$σ$ error).

astro-ph.GA↗

arXiv2018

Hubble Parameter and Baryon Acoustic Oscillation Measurement Constraints on the Hubble Constant, the Deviation from the Spatially-Flat $Λ$cdm Model, The Deceleration-Acceleration Transition Redshift, and Spatial Curvature

We compile a complete collection of reliable Hubble parameter $H(z)$ data to redshift $z \leq 2.36$ and use them with the Gaussian Process method to determine continuous $H(z)$ functions for various data subsets. From these continuous $H(z)$'s, summarizing across the data subsets considered, we find $H_0\sim 67 \pm 4\,\rm km/s/Mpc$, more consistent with the recent lower values determined using a variety of techniques. In most data subsets, we see a cosmological deceleration-acceleration transition at 2$σ$ significance, with the data subsets transition redshifts varying over $0.33<z_{\rm da}<1.0$ at 1$σ$ significance. We find that the flat-$Λ$CDM model is consistent with the $H(z)$ data to a $z$ of 1.5 to 2.0, depending on data subset considered, with 2$σ$ deviations from flat-$Λ$CDM above this redshift range. Using the continuous $H(z)$ with baryon acoustic oscillation distance-redshift observations, we constrain the current spatial curvature density parameter to be $Ω_{K0}=-0.03\pm0.21$, consistent with a flat universe, but the large error bar does not rule out small values of spatial curvature that are now under debate.

astro-ph.CO↗

arXiv2017

Median Statistics Estimate of the Distance to the Galactic Center

We show that error distributions of a compilation of 28 recent independent measurements of the distance from the Sun to the Galactic center, $R_{0}$, are wider than a standard Gaussian and best fit by an $n=4$ Student's $t$ probability density function. Given this non-Gaussianity, the results of our median statistics analysis, summarized as $R_{0}=8.0 \pm 0.3$ kpc ($2σ$ error), probably provides the most reliable estimate of $R_{0}$.

astro-ph.GA↗

arXiv2017

Inflation in a closed universe

To derive a power spectrum for energy density inhomogeneities in a closed universe, we study a spatially-closed inflation-modified hot big bang model whose evolutionary history is divided into three epochs: an early slowly-rolling scalar field inflation epoch and the usual radiation and non-relativistic matter epochs. (For our purposes it is not necessary to consider a final dark energy dominated epoch.) We derive general solutions of the relativistic linear perturbation equations in each epoch. The constants of integration in the inflation epoch solutions are determined from de Sitter invariant quantum-mechanical initial conditions in the Lorentzian section of the inflating closed de Sitter space derived from Hawking's prescription that the quantum state of the universe only include field configurations that are regular on the Euclidean (de Sitter) sphere section. The constants of integration in the radiation and matter epoch solutions are determined from joining conditions derived by requiring that the linear perturbation equations remain nonsingular at the transitions between epochs. The matter epoch power spectrum of gauge-invariant energy density inhomogeneities is not a power law, and depends on spatial wavenumber in the way expected for a generalization to the closed model of the standard flat-space scale-invariant power spectrum. The power spectrum we derive appears to differ from a number of other closed inflation model power spectra derived assuming different (presumably non de Sitter invariant) initial conditions.

astro-ph.CO↗

arXiv2016

Determining the Hubble constant from Hubble parameter measurements

We use 28 Hubble parameter, $H(z)$, measurements at intermediate redshifts $0.07 \leq z \leq 2.3$ to determine the present-day Hubble constant $H_0$ in four cosmological models. We measure $H_0 = 68.3^{ +2.7}_{ -2.6 }, 68.4^{ +2.9 }_{ -3.3 }, 65.0^{ +6.5 }_{ -6.6 }$ and $ 67.9^{ +2.4}_{-2.4}$ km s${}^{-1}$ Mpc${}^{-1}$ (1$σ$ errors) in the $Λ$CDM (spatially flat and non-flat), $ω$CDM and $ϕ$CDM models, respectively. These measured $H_0$ values are more consistent with the lower values determined from recent cosmic microwave background and baryon acoustic oscillation data, as well as with that found from a median statistics analysis of Huchra's compilation of $H_0$ measurements,but include the higher local measurements of $H_0$ within the 2$σ$ confidence limits.

astro-ph.CO↗

arXiv2016

Hubble parameter measurement constraints on the redshift of the deceleration-acceleration transition, dynamical dark energy, and space curvature

We compile an updated list of 38 measurements of the Hubble parameter $H(z)$ between redshifts $0.07 \leq z \leq 2.36$ and use them to place constraints on model parameters of constant and time-varying dark energy cosmological models, both spatially flat and curved. We use five models to measure the redshift of the cosmological deceleration-acceleration transition, $z_{\rm da}$, from these $H(z)$ data. Within the error bars, the measured $z_{\rm da}$ are insensitive to the model used, depending only on the value assumed for the Hubble constant $H_0$. The weighted mean of our measurements is $z_{\rm da} = 0.72 \pm 0.05\ (0.84 \pm 0.03)$ for $H_0 = 68 \pm 2.8\ (73.24 \pm 1.74)$ km s$^{-1}$ Mpc$^{-1}$ and should provide a reasonably model-independent estimate of this cosmological parameter. The $H(z)$ data are consistent with the standard spatially-flat $Λ$CDM cosmological model but do not rule out non-flat models or dynamical dark energy models.

astro-ph.CO↗

arXiv2016

Constraints on non-flat cosmologies with massive neutrinos after Planck 2015

We investigate two dark energy cosmological models (i.e., the $Λ$CDM and $ϕ$CDM models) with massive neutrinos assuming two different neutrino mass hierarchies in both the spatially flat and non-flat scenarios, where in the $ϕ$CDM model the scalar field possesses an inverse power-law potential, $V(ϕ)\propto ϕ^{-α}$ ($α>0$). Cosmic microwave background data from Planck 2015, baryon acoustic oscillations data from 6dFGS, SDSS-MGS, BOSS-LOWZ and BOSS CMASS-DR11, the JLA compilation of Type Ia supernova apparent magnitude observations, and the Hubble Space Telescope $H_0$ prior, are jointly employed to constrain the model parameters. We first determine constraints assuming three species of degenerate massive neutrinos. In the spatially flat (non-flat) $Λ$CDM model, the sum of neutrino masses is bounded as $Σm_ν < 0.165 (0.299)$ eV at 95% confidence level (CL). Correspondingly, in the flat (non-flat) $ϕ$CDM model, we find $Σm_ν < 0.164 (0.301)$ eV at 95% CL. The inclusion of spatial curvature as a free parameter results in a significant broadening of confidence regions for $Σm_ν$ and other parameters. In the scenario where the total neutrino mass is dominated by the heaviest neutrino mass eigenstate, we can obtain the similar conclusions as those obtained in the degenerate neutrino mass scenario. In addition, the results show that the bounds on $Σm_ν$ based on two different neutrino mass hierarchies have insignificant differences in the spatially flat case for both the $Λ$CDM and $ϕ$CDM models, however, the corresponding differences are larger in the non-flat case.

astro-ph.CO↗

arXiv2015

Non-Gaussian Error Distributions of LMC Distance Moduli Measurements

We construct error distributions for a compilation of 232 Large Magellanic Cloud (LMC) distance moduli values from de Grijs (2014) that give an LMC distance modulus of (m-M)_{0}=18.49 plus/minus 0.13 mag (median and 1 sigma symmetrized error). Central estimates found from weighted mean and median statistics are used to construct the error distributions. The weighted mean error distribution is non-Gaussian --- flatter and broader than Gaussian --- with more (less) probability in the tails (center) than is predicted by a Gaussian distribution; this could be the consequence of unaccounted-for systematic uncertainties. The median statistics error distribution, which does not make use of the individual measurement errors, is also non-Gaussian --- more peaked than Gaussian --- with less (more) probability in the tails (center) than is predicted by a Gaussian distribution; this could be the consequence of publication bias and/or the non-independence of the measurements. We also construct the error distributions of 247 SMC distance moduli values from de Grijs (2015). We find a central estimate of (m-M)_{0}=18.94 plus/minus 0.14 mag (median and 1 sigma symmetrized error), and similar probabilities for the error distributions.

astro-ph.CO↗

arXiv2015

Non-Gaussian Error Distribution of $\rm{^{7}Li}$ Abundance Measurements

We construct the error distribution of $\rm{^{7}Li}$ abundance measurements for 66 observations (with error bars) used by Spite12 that give $\rm{A(Li)}=2.21 \pm 0.065$ (median and 1$σ$ symmetrized error). This error distribution is somewhat non-Gaussian, with larger probability in the tails than is predicted by a Gaussian distribution. The 95.4% confidence limits are 3.0$σ$ in terms of the quoted errors. We fit the data to four commonly used distributions: Gaussian, Cauchy, Student's t, and double exponential with the center of the distribution found with both weighted mean and median statistics. It is reasonably well described by a widened $n=8$ Student's $t$ distribution. Assuming Gaussianity, the observed A(Li) is 6.5$σ$ away from that expected from standard Big Bang nucleosynthesis given the $Planck$ observations Coc2014. Accounting for the non-Gaussianity of the observed A(Li) error distribution reduces the discrepancy to 4.9$σ$, which is still significant.

astro-ph.CO↗

arXiv2014

Cosmological constraints from large-scale structure growth rate measurements

We compile a list of $14$ independent measurements of large-scale structure growth rate between redshifts $0.067 \leq z \leq 0.8$ and use this to place constraints on model parameters of constant and time-evolving general-relativistic dark energy cosmologies. With the assumption that gravity is well-modeled by general relativity, we discover that growth-rate data provide restrictive cosmological parameter constraints. In combination with type Ia supernova apparent magnitude versus redshift data and Hubble parameter measurements, the growth rate data are consistent with the standard spatially-flat $Λ$CDM model, as well as with mildly evolving dark energy density cosmological models.

astro-ph.CO↗

arXiv2014

Observational constraints on non-flat dynamical dark energy cosmological models

We constrain two non-flat time-evolving dark energy cosmological models by using Hubble parameter data, Type Ia supernova apparent magnitude measurements, and baryonic acoustic oscillation peak length scale observations. The inclusion of space curvature as a free parameter in the analysis results in a significant broadening of the allowed range of values of the parameter that governs the time evolution of the dark energy density in these models. While consistent with the "standard" spatially-flat $Λ$CDM cosmological model, these data are also consistent with a range of mildly non-flat, slowly time-varying dark energy models. After marginalizing over all other parameters, these data require the averaged magnitude of the curvature density parameter $|Ω_{k0}| \lesssim 0.15$ at 1$σ$ confidence.

astro-ph.CO↗

arXiv2014

Cosmographic bounds on the cosmological deceleration-acceleration transition redshift in $f(\mathcal{R})$ gravity

We examine the observational viability of a class of $f(\mathcal{R})$ gravity cosmological models. Particular attention is devoted to constraints from the recent observational determination of the redshift of the cosmological deceleration-acceleration transition. Making use of the fact that the Ricci scalar is a function of redshift $z$ in these models, $\mathcal {R=R}(z)$, and so is $f(z)$, we use cosmography to relate a $f(z)$ test function evaluated at higher $z$ to late-time cosmographic bounds. First, we consider a model independent procedure to build up a numerical $f(z)$ by requiring that at $z=0$ the corresponding cosmological model reduces to standard $Λ$CDM. We then infer late-time observational constraints on $f(z)$ in terms of bounds on the Taylor expansion cosmographic coefficients. In doing so we parameterize possible departures from the standard $Λ$CDM model in terms of a two-parameter logarithmic correction. The physical meaning of the two parameters is also discussed in terms of the post Newtonian approximation. Second, we provide numerical estimates of the cosmographic series terms by using Type Ia supernova apparent magnitude data and Hubble parameter measurements. Finally, we use these estimates to bound the two parameters of the logarithmic correction. We find that the deceleration parameter in our model changes sign at a redshift consistent with what is observed.

gr-qc↗

arXiv2013

Nonflat time-variable dark energy cosmology

We generalize the time-variable dark energy scalar field $Φ$ model ($Φ$CDM) to nonflat space. We show that even in the space-curvature-dominated epoch the scalar field solution is a time-dependent fixed point or attractor, with scalar field energy density that grows relative to the energy density in spatial curvature. This is the first example of a physically consistent and complete model of dynamical dark energy in a nonflat geometry.

astro-ph.CO↗

arXiv2013

Median statistics cosmological parameter values

We present median statistics central values and ranges for 12 cosmological parameters, using 582 measurements (published during 1990-2010) collected by Croft & Dailey (2011). On comparing to the recent Planck collaboration Ade et al. 2013 estimates of 11 of these parameters, we find good consistency in nine cases.

astro-ph.CO↗

arXiv2013

Binned Hubble parameter measurements and the cosmological deceleration-acceleration transition

Weighted mean and median statistics techniques are used to combine 23 independent lower redshift, $z<1.04$, Hubble parameter, $H(z)$, measurements and determine binned forms of $H(z)$. When these are combined with 5 higher redshift, $1.3\leqslant z \leqslant 2.3$, $H(z)$ measurements the resulting constraints on cosmological parameters, of three cosmological models, that follow from the weighted-mean binned data are almost identical to those derived from analyses using the 28 independent $H(z)$ measurements. This is consistent with what is expected if the lower redshift measurements errors are Gaussian. Plots of the binned weighted-mean $H(z)/(1+z)$ versus $z$ data are consistent with the presence of a cosmological deceleration-acceleration transition at redshift $z_{\rm da}=0.74 \pm 0.05$ \citep{farooq3}, which is expected in cosmological models with present-epoch energy budget dominated by dark energy as in the standard spatially-flat $Λ$CDM cosmological model.

astro-ph.CO↗